The formation of ice on aircraft wing and/or control surfaces can impair the aerodynamic characteristics of the aircraft and, in some cases, even make it extremely difficult to control.
Ice normally forms on aircraft wing and/or control surfaces as the aircraft flies through air containing water droplets, which have remained liquid at temperatures below zero degrees Centigrade, and which are commonly known as supercooled water droplets.
Supercooled water droplets tend to solidify in contact with, and so form a layer of supercooled ice on, aircraft wing and/or control surfaces.
Encountering supercooled water droplets of a diameter below a given, say, 50 micron threshold does not normally pose a serious risk to the aircraft.
This is due to the aircraft wing and/or control surfaces disturbing the airflow, and to the fact that, possessing little motion, most of the water droplets below the threshold diameter tend to pass over, as opposed to striking, the aircraft wing and/or control surfaces. It has been observed that only a small part of the water droplets below the threshold diameter actually come into contact with the leading edges of wing and/or control surfaces.
To remove ice forming on the leading edges of wing and/or control surfaces, aircraft are therefore equipped with ice detecting and deicing devices.
Supercooled water droplets above the threshold diameter, on the other hand, possess greater inertia and so move along paths which are not disturbed to a great extent by interaction of the aircraft wing and/or control surfaces with the air.
As a result, supercooled water droplets above the threshold diameter tend to strike the aircraft wing and/or control surfaces, and so also form ice on parts of the wing and/or control surfaces other than the leading edges, thus endangering the aircraft.
Devices for detecting ice caused by supercooled water droplets above the threshold diameter are known, such as the one described in U.S. Pat. No. 6,296,320.
This substantially comprises a casing fixed to the fuselage; a sensor, in particular a vibrating member, housed in a cavity in the casing; and an element for measuring the vibration frequency of the sensor.
The detecting device also comprises a conduit formed in the casing and having an airflow inlet; and an outlet upstream from the sensor in the airflow direction with respect to the device.
The airflow in the conduit creates vortices about the sensor. Possessing little motion, the supercooled water droplets below the threshold diameter are unable to penetrate the vortices, with the result that most of them fail to come into contact with the sensor. The supercooled water droplets above the threshold diameter, on the other hand, possess sufficient motion to penetrate the vortices and strike and form ice on the sensor. The ice formed alters the natural vibration frequency of the sensor, which is detected by the measuring element.
Another example of a detecting device is described in US-A-2002/0158768, and substantially comprises a first and second ice-sensitive probe; a first and second excitation circuit for exciting the first and second probe respectively; and a device for measuring the natural vibration frequencies of the first and second probe.
The detecting device is designed so that supercooled water droplets above the threshold diameter strike the first probe, whereas those below the threshold diameter and possessing little motion are diverted and do not strike the first probe.
The detecting device is also designed so that both supercooled water droplets above and below the threshold diameter strike, and so form ice on, the second probe.
The first and second excitation circuit excite the first and second probe respectively, and the measuring device determines the first natural vibration frequency of the first probe, on which ice is formed by striking supercooled water droplets above the threshold diameter, and the second natural vibration frequency of the second probe, on which ice is formed by striking supercooled water droplets both below and above the threshold diameter.
From the ratio between the first and second natural vibration frequency, it is possible to determine the presence of supercooled water droplets in the air through which the aircraft is moving.
The above detecting devices are substantially based on determining the natural frequency of a sensor, on which ice is formed by striking supercooled water droplets above the threshold diameter, which means these sensors require an excitation circuit to excite them, and a measuring circuit for measuring their natural frequency.
A need is felt within the industry to equip aircraft with detecting devices capable of rapidly detecting the presence of supercooled water droplets above the threshold diameter, without recourse to the above excitation and measuring circuits.